In general, spatial multiplexing is a scheme for increasing the data rate through use of spatial diversity between divided transmission/reception antennas. In uplink, it is difficult to utilize transmit antenna diversity due to various restrictions. In order to solve such a problem, a Collaborative Spatial Multiplexing (C-SM) mode for obtaining an effect similar to that of the conventional transmit antenna diversity through use of collaboration between terminals is disclosed in IEEE 802.16. In the C-SM mode, when each of the two terminals has one transmission antenna, the two terminals are allocated the same frequency resource and transmit data at the same time. In such a C-SM mode, the two terminals output pilot signals through code multiplexing, wherein the two terminals can output two different data streams.
FIG. 1 is a view illustrating a pilot pattern when the general C-SM mode is used.
A first terminal using pilot pattern A and a second terminal using pilot pattern B share one tile (that means a resource unit having a predetermined size) with each other. The two terminals use pilot tones constituted by at least one pilot subcarrier on a division basis while sharing and using data tones constituted by at least one data subcarrier. Accordingly, although the data rate that one terminal can obtain does not change, the throughput increases from the viewpoint of a sector.
A significant characteristic of a method of allocating a burst to an uplink frame using the C-SM mode, as compared with using only a single-input multiple-output (SIMO) mode, is that an uplink burst is allocated for each layer using two pilot patterns. This characteristic is restricted when a terminal cannot use both pilot patterns of one slot, when a terminal cannot use one slot in the SIMO mode and the C-SM mode at the same time, and when a terminal uses only the SIMO mode from necessity.
According to IEEE 802.16, methods for allocating a C-SM burst to a frame are roughly classified into two methods.
The first method is to allocate a burst using pilot pattern A and a burst using pilot pattern B to both sides through use of a Multiple-Input-Multiple-Output Uplink Basic Information Element (MIMO UL Basic IE). This method has disadvantages in that a part of the slots is wasted because of the a difference in size between the burst using pilot pattern A and the burst using pilot pattern B, and it is not easy to make both sides effective.
The second method is to separately allocate a burst using pilot pattern A and a burst using pilot pattern B one by one through use of a UL H-ARQ Chase Sub-Burst information element.
Of these methods, particularly, a method for allocating an uplink burst through use of the C-SM mode requires two functions.
First, in order to maintain efficient Rise-over-Thermal (RoT) at a predetermined level, an interference control function of controlling a rate of slots using the C-SM mode to be proper is required. Generally, with respect to the same number of slots, using the slots in the C-SM mode causes greater interference to neighboring cells than using the slots in the SIMO mode. For this reason, one pilot pattern may be used in the same manner as when only the SIMO mode is used, while the other pilot pattern is used only when a specific condition is satisfied. According to the conventional method, an additional pilot pattern is used only for terminals very near to the base station so as to minimize the increase in the efficient RoT as much as possible. In this case, a part of the slots may use one pilot pattern while the other slots use both pilot patterns. The slots using both pilot patterns require greater uplink transmission power than the slots using only one pilot pattern. Meanwhile, with respect to a terminal located in the boundary of a cell, it may be necessary to prevent the terminal from using both pilot patterns in order to ensure coverage. According to the conventional method, since it is not easy to control a specific burst to use one pilot pattern, a separate consideration is required in order to complement the coverage problem. A simple solution to prevent a terminal located in a cell boundary from using both pilot patterns is to make the terminal use the SIMO mode. Since interference is influenced by the conditions for use with the SIMO mode, the conventional interference control scheme needs to be modified.
Second, it must be possible to efficiently add a burst with a specific size. Even in the case of using only the C-SM mode, a newly selected current burst may, or may not, be added according to the disposed position of a pre-allocated burst. Such a result is very undesirable when the newly selected burst includes a connection having relay-related Quality-of-Service (QoS) requirements. In order to avoid such a problem, a function of rearranging the disposition of the pre-allocated burst before the newly selected burst is added to a frame is required. In addition, when the SIMO mode is used together with the C-SM mode, the problem becomes more complex. A burst including a connection selected according to the scheduling priorities may use the SIMO mode or the C-SM mode. That is, bursts using the SIMO mode and bursts using the C-SM mode are alternately selected. However, since a scheme of disposing a pre-allocated burst in order to add a burst using the SIMO mode is different from a scheme of disposing a pre-allocated burst in order to add a burst using the C-SM mode, it is necessary to develop supplementary technology for adding the bursts.